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Greenpeace Japan on Thursday said it has discovered radioactive contamination in Fukushima's riverbanks, estuaries and coastal waters at a scale hundreds of times higher than pre-2011 levels.

One sample of sediment taken along the Niida River, less than 30 km northwest of the crippled Fukushima No. 1 power plant, revealed the presence of cesium-134 and cesium-137 at levels of 29,800 becquerels per kilogram.

That was just one of 19 samples of dried sediment and soil the environmental activist group took and analyzed from the banks of the Abukuma, Niida, and Ota rivers. The samples were collected by Greenpeace in February and March.

All of the samples but one exhibited more than 1,000 Bq/kg of radioactive material. The lowest level, 309 Bq/kg, was logged at a spot along the Abukuma River.

Cesium-134 has a half-life of about two years, but cesium-137 has a half-life of 30 years and is considered particularly hazardous. The standard limits set for radioactive cesium in Japan are 100 Bq/kg for general foods and 10 Bq/kg for drinking water.

"The radiological impacts of the Fukushima nuclear disaster on the marine environment, with consequences for both human and nonhuman health, are not only the first years. They are both ongoing and future threats, principally the continued releases from the Fukushima No. 1 plant itself and translocation of land-based contamination throughout Fukushima Prefecture, including upland forests, rivers, lakes and coastal estuaries," the report said.

Greenpeace Japan also published the results of tests on dried marine sediment samples collected at 25 points off the Fukushima coastal area, including three river estuaries, during this same period, at depths of between 7.4 and 30.6 meters. The results showed that the highest level of cesium was 144 Bq/kg taken from a sample collected off the coast from the Fukushima power plant, while the lowest total cesium figure was 6.5 Bq/kg off Nakanosaku, well to the south of the plant.

In addition to Fukushima, Greenpeace Japan took dried sediment samples from Lake Biwa at three locations near the shore. The results showed cesium levels to be between 7.1 Bq/kg and 13 Bq/kg at two locations, and negligible at the other two.

The safety of Lake Biwa, which provides drinking water for about 14 million people in the Kansai region, has become a major bone of contention between Kansai Electric Power Co., which wants to restart reactors in neighboring Fukui Prefecture, and residents in and around Lake Biwa who are fighting to keep them shut down.